Mirrors are made by the successive application of silver and copper films to a glass substrate. The copper film is then protected by a protective paint layer to protect the metal films against degradation due to attack by moisture or contaminants in the atmosphere.
The metal films are liable to degradation due to oxidation and heat and to attack by contaminants in the atmosphere. The durability of the metal films is tested by standard industry tests, such as a salt spray test and a copper accelerated salt spray test (known as the CASS test).
It has been found that the durability of the resulting mirror is greatly enhanced if the paint layer is lead based. The explanation for this phenomenon is not clear but it seems that the lead compounds in the coating, although being only sparingly soluble, may act in two ways:
1. to "poison" the copper/silver cell and prevent electron transfer and hence destruction of the layers, or
2. to act as a sacrificial electrode leaving the copper/silver layers unaffected.
If either of these explanations is correct one would expect other commonly available metals to provide the same protective effect. Many commonly available metals were tested and a number of rare metals also which from a consideration of their position in the electrochemical series should work. However, no metals other than lead provided the same protection.
The incorporation of lead based compounds in mirror backings is undesirable and the invention commenced with the aim of identifying a protective non-lead compound which could be used as an effective mirror backing. UK Patent Specification No. 1074076 discloses the use of substituted azoles to protect mirror backings but the applicants have found that these compounds do not perform well in the standard industry tests.